Diffusion and Reaction of Hydrogen on Rutile TiO2(011)-2×1: The

Article pubs.acs.org/JPCC

Diffusion and Reaction of Hydrogen on Rutile TiO2(011)-2×1: The Role of Surface Structure Junguang Tao,†,§ Qian Cuan,‡,§ Xue-Qing Gong,*,‡ and Matthias Batzill*,† †

Department of Physics, University of South Florida, Tampa Florida 33620, United States State Key Laboratory of Chemical Engineering, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P.R. China.

‡

ABSTRACT: Hydrogen adsorption and reaction on the rutile TiO2(011)-2×1 has been investigated by a combination of high-resolution scanning tunneling microscopy and density functional theory calculations. Hydroxyl formation on the reconstructed surface is weak, and hydroxyls have only been observed on one of the three different surface oxygen sites. Recombination of hydrogen and desorption of H2 is prevented by a large kinetic barrier. Instead, hydrogen is removed from the surface at elevated temperature by diffusion into the bulk. This is contrasted with photoinduced processes investigated by UV−irradiation under ultra high vacuum conditions, which leads to desorption of hydrogen from the surface, indicating a photoinduced lowering of the reaction barrier. Our studies are also compared to previous studies on the rutile TiO2(110) surface where different thermal and photoinduced processes have been reported. These differences are explained by three competing reaction pathways: (i) bulk diffusion, (ii) H2 recombination, and (iii) water formation at the surface by lattice oxygen abstraction. The dependence of the reaction on the hydrogen-adsorption energies as well as on kinetic diffusion and reaction barriers and pathways can explain the observed differences between these two surface orientations. rutile (110) surface16−18 and computational studies were also performed on anatase surfaces.19 In these studies only formation of hydroxyls, i.e., adsorption of hydrogen on oxygen atoms, has been reported, while formation of hydride (Ti−H) has been excluded. In the current work, we report the adsorption and reaction of hydrogen on the rutile TiO2(011) surface. In vacuum this surface orientation forms a 2×1 surface reconstruction.20−22 This reconstructed surface is much more thermodynamically stable by reducing unsaturated or “dangling” bonds than the bulk truncation. However, the absence of truncated bonds at the surface makes it also a much less chemically active surface than, e.g., the more frequently studied rutile (110) surface. The 2×1 reconstructed TiO2(011) surface exhibits two different kinds of 2-fold coordinated oxygen atoms that both are potential hydrogen adsorption sites (see Figure 1). However, compared to the TiO2(110) surface where the 2fold oxygen sites are the result of bulk truncation and thus exhibit “broken” bonds, the adsorption on the (011)-2×1 surface may be expected to be much weaker. Consequently, we anticipate different hydrogen chemistry on the rutile (011)-2×1 than on most other bulk-truncated TiO2 surfaces. Experiments on the rutile (110) surface have shown that the surface can be (partially) hydroxylated by exposure to atomic hydrogen in UHV. The reported saturation coverages measured by scanning tunneling microscopy (STM) vary widely between

1. INTRODUCTION Titanium dioxide (TiO2) is a model system for surface properties of transition metal oxides.1 Chemical surface properties play a critical role in photocatalysis for which TiO2 is also the prototypical material.2,3 One particular exciting application for TiO2 photocatalysis is the production of chemical fuels directly from sunlight. The most commonly studied reaction in this respect is the photochemical splitting of water over TiO2 electrodes.4 In this application in particular, and in other chemical and environmental applications as well, the formation, stability, and reaction of hydroxyls at the surface are of paramount importance to describing surface processes. It is well documented that photocatalytic activity markedly varies for different crystallographic surface orientations of the same material.5−10 This may be partially explained by bulk11,12 and surface electronic properties,13−15 but variations in surface chemical properties are likely to play a very important role, too. This study illustrates the challenges in correctly describing these chemical surface properties. Both thermodynamic and kinetic adsorption and reaction properties have to be taken into account to describe our experimentally observed surface processes accurately. Our results show very different thermal and photo reactions for adsorbed hydrogen on the rutile (011)2×1 surface compared to the previously reported properties of the (110) surface and thus verifying the importance of chemical properties to describe surface dependent reactions of an important adsorbate for photocatalysis. Hydrogen adsorption on TiO2 has been previously studied by ultrahigh vacuum (UHV) surface science techniques on the © 2012 American Chemical Society

Received: June 30, 2012 Revised: September 4, 2012 Published: September 4, 2012 20438

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Figure 1. Surface structures of TiO2(011)-2×1. Calculated structures of reconstructured TiO2(011)-2×1 surface from the side (a) and top (b) view against the [001] direction. The top TiO2 layer is illustrated in ball-and-stick mode, and the rest of the surface slab is in line mode. The Ti atoms are in gray and O in red. The light blue circles in panel b illustrate the [001] channel through the whole bulk. (c) High-resolution STM images of clean rutile TiO2(011)-2×1 surface. The inset of panel c highlights the adsorption position of surface hydroxyl. Panel d shows a surface with more hydroxyls and part of the image, indicated by the blue rectangular, is enlarged in panel e to show the separation of the hydroxyls.

0.2516 and 0.7 ML.17 In another study, reaction of the (110) surface with carboxylic acids and subsequent selective removal of the carboxylate by photoreaction allowed the preparation of a surface with ∼0.5 ML hydroxyl coverage.18 Importantly, this study implies that hydrogen is not being desorbed in UHV by UV-irradiation. There also exists some controversy regarding the thermal chemistry of the hydroxyls with one report not observing any desorption of H2 or water and therefore suggesting hydrogen diffuses into the bulk upon annealing of the sample,17 while some other study observed water desorption with 100% yield and thus suggesting that hydrogen reacts with lattice oxygen to desorb as water.18 Both experimental studies supported their finding by density functional theory (DFT) calculations. Studies by Du et al.18 found that H2 recombinative desorption is only limited by kinetic barriers but would be thermodynamically accessible, whereas water desorption, by abstraction of lattice oxygen on the other hand, is barrierless but only becomes thermodynamically favorable above 500 K, in agreement with their experiments. Hydrogen diffusion into the bulk exhibits a ∼ 1.2 eV kinetic barrier18,23,24 and hydrogen in the bulk is ∼0.4 eV less stable than surface hydroxyls25 on the (110) surface. The presence of a low concentration of subsurface hydrogen on the (110) surface was also indicated by STM measurements.25 The apparent sensitive dependence of the reaction and diffusion of hydrogen on kinetic barriers imply that there should be a pronounced structure dependence of the hydrogen chemistry over TiO2 surfaces. Computational studies of hydrogen on anatase TiO2 (101) surfaces further illustrate this structure dependence on the diffusion and reaction barriers for hydrogen.19 On the anatase surface it was concluded that hydrogen diffusion into the bulk was kinetically the most favorable process. The rutile TiO2(011)-2×1 surface provides another welldefined titania surface with a different surface structure that allows both experimental and computational investigation of hydrogen adsorption. Since the adsorption energy of hydrogen on the (011)-2×1 surface is expected to be smaller than that for the rutile (110) surface, different reaction behavior of hydrogen

may be observed. Our earlier studies on thermal reaction of acetate also indicated a lack of desorption of hydrogen species (H2 or water) which we had interpreted as hydrogen diffusion into the bulk.26 The present study was performed to clarify the thermal behavior of adsorbed hydrogen. In addition, we compare thermal to UV-light induced surface reaction of hydrogen. In a combination of STM experiments and DFT calculations, it is found that adsorbed hydrogen disappears from the surface at elevated temperatures (∼500 K). Surprisingly, hydrogen reappears at the surface upon cooling of the sample, indicating that it has not been removed from the sample by desorption. The lack of thermal desorption is in agreement with our estimation for the kinetic reaction barrier for hydrogen recombination. On the other hand, we find experimentally that UV light induced excitation of hydrogen enables desorption of hydrogen from the (011)-2×1 surface but not from the (110) surface.

2. EXPERIMENTAL AND COMPUTATIONAL APPROACHES The STM measurements were performed in a UHV chamber with base pressure in the low 10−10 Torr range. The TiO2 single crystal was cleaned by multiple cycles of Ar+ sputtering at room temperature (1 kV, 5 μA, 30 min), followed by annealing at 650 °C for 15 min. In the STM chamber the TiO2 crystal was clipmounted to a tantalum (Ta) sample plate and the sample heater temperature was calibrated with a chromel-alumel thermocouple spot-welded to the Ta sample plate. STM imaging was done in constant current mode using an electrochemically etched tungsten tip. Empty-states STM images are shown throughout the paper. The tunneling bias is typically 1.2 V and the current is 0.3 nA. Exposure of the sample to atomic hydrogen was carried out by backfilling the chamber with 2 × 10−6 Torr H2 through a precision leak valve and by dissociating it on a hot tungsten filament in close proximity to the sample surface. For studying photoinduced surface reactions, a 100 W Hg arc lamp was used. The UV light was coupled into a 0.6 mm fused silica optical fiber. The optical 20439

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Figure 2. Surface evolution with increasing atomic hydrogen exposure. STM images of clean (a), after nominal exposure of 1200 Langmuir (L = 1 × 10−6 Torr·s) (b), and 14400 L hydrogen (c). Panel d shows a plot of hydroxyl concentration as function of the hydrogen exposure.

fiber was brought through a UHV optical fiber feedthrough, to about ∼1 cm in front of the TiO2 sample surface. To avoid any heating effects on the sample, a water filter was used to remove infrared radiation from the emitted light. The total energy DFT calculations have been carried out within the generalized gradient approximation (GGA) using the PWScf code included in the Quantum-Espresso package27 Electron−ion interactions were described by ultrasoft pseudopotentials,28 with electrons from O 2s, 2p and Ti 3s, 3p, 3d, 4s shells explicitly included in the calculations. Plane-wave basis set cut-offs for the smooth part of the wave functions and the augmented density were 25 and 200 Ry, respectively. The rutile TiO2 bulk was modeled using a 2 × 2 × 3 super cell (9.172 × 9.172 × 8.847 Å3), which contains 24 TiO2 units, and corresponding 2 × 2 × 2 k-point mesh was used. All of the atoms in the bulk were allowed to relax during calculations. The rutile TiO2(011)-2×1 surface was modeled as periodic slabs with 5 layers of oxide, and the vacuum between slabs was more than 10 Å. The 2 × 2 surface cell (9.172 × 10.905 Å2) and corresponding 1 × 1 × 1 k-point mesh were used in the calculations. The reconstruction and adsorption were modeled on one side of the slab, and during structural optimizations, all of the atoms, except those in the bottom TiO2 layer of the slab, were allowed to move (force threshold was 0.05 eV/Å). To estimate the adsorption energies Ead of H, the following expressions were considered:

cell; ETiO2is the total energy of the TiO2(011)-2×1 slab; EH2 is the total energy of a single H2 molecule in gas-phase. All the calculations were conducted without spin-polarization, and for adsorption of a single H, the difference between adsorption energies obtained with and without spin-polarization was estimated to be below 0.01 eV. A climbing image nudged elastic band (CI-NEB) method was employed to locate the transition states of various H transfer and reaction pathways.29 Considering the rather short distance for H to move from one oxygen to another, only three images were chosen in the calculations. A few testing calculations with five images were also conducted, and no difference for the structures or barriers of transition states was observed.

3. EXPERIMENTAL RESULTS The crystal structure of TiO2(011)-2×1 reconstruction is shown in Figure 1a,b in side- and top-view modes, respectively. Typical empty state STM images of TiO2(011)-2×1 exhibit a zigzag pattern20 as for example shown in Figure 1c. The periodicity of this pattern corresponds to the 2×1 surface unit cell. This appearance of the (011)-2×1 surface in STM is the result of a convolution of electronic and topographic effects. Recent DFT simulations of the STM image suggested that Ti3d states of the topmost Ti-atoms predominately give rise to the zigzag pattern,30 i.e., the surface Ti-sites are imaged as bright protrusions. STM images of some of our samples exhibit additional bright spots at sites in between the bright protrusions of the zigzag substrate pattern, which is illustrated in the inset of Figure 1c. Using the assignment that the bright protrusions in the STM images of the clean TiO2(011)-2×1 surface corresponds to Ti-sites, we can infer that the bright spots are

Ead = − [E H/TiO2 − E TiO2 − 1/2E H2] or Ead = − [E2H/TiO2 − E TiO2 − E H2] × 1/2

in which EH/TiO2 (E2H/TiO2) is the total energy of the interacting system containing one (two) H and TiO2 support in a surface 20440

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Figure 3. Temperature dependence of surface hydroxyls. STM images of vacuum prepared sample at room temperature with high OH concentration (a), the same sample imaged at 500 K (b), and cooled down to room temperature (c). All images are 20 × 20 nm2.

exposure. Initially the coverage increases linearly with exposure time but it appears to saturate at a coverage of ∼0.22 monolayer (ML), where 1 ML is defined as the density of (011)-1×1 units, i.e., 3.98 × 1014 cm−2. The STM images show that the hydroxyls are randomly distributed at the surface without any long-range ordering. However, we also never observed two hydroxyls to be adsorbed in the closest possible arrangement, i.e., at neighboring 2-fold top oxygen sites. The closest observed hydroxyls are on next nearest top oxygen sites. A similar saturation coverage was found for the (110) surface using the same method for hydrogen production.16 However, in later studies a much higher hydroxyl coverage has been reported for the (110) surface by using much higher hydrogen exposures.23 The behavior of hydroxyls as a function of sample temperature was investigated by high temperature STM studies. The behavior of hydroxyls was independent of their preparation, i.e., atomic hydrogen adsorbed from the gas phase behaved identical to hydrogen originating from bulk diffusion. Figure 3 shows three STM images of the same sample at different temperatures. Figure 3a shows the initial sample at room temperature with hydroxyl coverage of around 0.12 ML. Once this sample was heated up in the STM and measured at ∼500 K, only very few hydroxyls were observed at the surface. The fact that the remaining hydroxyls are still stationary and thus can be imaged with STM indicates that it is not fast surface diffusion that makes the hydroxyls invisible in high temperature STM, but rather that most of the hydroxyls have disappeared from the surface. Surprisingly, after we cooled the sample back down to room temperature the hydroxyls reappeared at the surface and we estimated the coverage to 0.10 ML, i.e., close to that of the initial coverage. The slightly lower coverage after annealing suggests that some hydrogen remains in the bulk. However, different cooling rates from ∼1 to ∼10 K/s did not make a measurable difference in the hydrogen coverage at the surface suggesting that the hydrogen out diffusion is much faster than the achievable cooling rates. Since we never observed spontaneous formation of hydroxyls on a clean (011)-2×1 surface in vacuum (unlike for the (110) surface where hydroxyls may form by dissociating residual water at surface defects), the reappearance of hydroxyls upon cooling suggests that the hydroxyls were not desorbed from the sample upon heating, but rather diffused into the subsurface region at elevated temperature. Thus the experiments suggest a reversible diffusion of hydrogen from the surface into the bulk in a temperature cycle. Finally, we also have investigated photoreaction of hydroxyls by UV irradiation. Figure 4 shows STM images of (a) a

situated close to the 2-fold top oxygen sites (see Figure 1a). Previous reports indicate that oxygen vacancies (Vo) are rarely formed on the TiO2(011)-2×1 surface31 and therefore it is more likely that the bright spots are due to adsorbates, in particular hydrogen. From analogy with the (110) surface, hydrogen at the OH sites are expected to appear bright under empty state imaging conditions.16 It has also been shown on the (110) surface that hydroxyls can react with oxygen at room temperature32 and thus oxygen exposure can lead to a hydroxyl free surface. We have observed the same for the (011) surface, i.e., exposure of the surface to 10−6 Torr O2 at room temperature for 30 min results in the removal of the bright spots from the surface. This is additional evidence that the adsorbates we observed are hydrogen, indicating that some (011)-2×1 samples can have spontaneous hydrogen adsorption at the surface. However, unlike for the (110) samples where hydroxyls are commonly observed on vacuum prepared samples due to water dissociation at oxygen vacancies, the (011)-2×1 surface usually remains hydroxyl free under UHV conditions for extended time periods. Exposure to high doses of water has, however, shown in the past to cause hydroxyls to form at the surface at room temperature.33,34 Nevertheless, in our case, we believe the hydroxyls observed on the (011)-2×1 surface originate from outward diffusion of hydrogen that was trapped in the bulk. Diffusion of hydrogen in such a way has been previously observed for other oxides.35−37 Figure 1d shows a surface area with a higher concentration of hydroxyls. Close inspection shows that the hydroxyls are randomly located either slightly to the right or left side of the center-line of the zigzag rows. A separation of ∼1 Å was measured between these two kinds of protrusions in the [100] direction (normal to the zigzag rows), as indicated in Figure 1e. This agrees well with the separation of top O atoms of ∼0.9 Å on the TiO2(011)-2×1 structural model.20,22 These STM results suggest that the top 2-fold O atoms are more reactive in term of OH formation than the 2-fold O atoms bridging the top Ti-atoms with the trough Ti-atoms (we call these sites bridging oxygen in the following). This agrees with the expectation that the Brønsted acidity of the top O is stronger,22 and therefore, it can form hydroxyls more readily. Instead of relying on hydrogen to be present at the surface due to diffusion from the bulk, we also investigated the adsorption of hydrogen from the gas phase. Exposing the surface to H2 does not increase the hydroxyl coverage because of the absence of any sites that could promote H2 scission. Therefore we exposed the sample to atomic hydrogen by cracking hydrogen at a hot tungsten filament. Figure 2 shows the evolution of the surface with increasing atomic hydrogen 20441

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Figure 4. Photoreactions of hydroxyls. STM images of vacuum prepared hydroxylated rutile TiO2(011)-2 × 1 surface (a) and after UV irradiation of 90 min (b). Both images are 20 × 20 nm2.

Figure 5. Calculated structures (side-view) of one H adsorbed on the top (a), bridging (b), and trough O (c) of the reconstructed rutile TiO2(011)2×1 surface. The H is in white. The values in brackets are H adsorption energies estimated with respect to a gas-phase atomic H.

Figure 6. Calculated structures (top view against [001] direction) of two coadsorbed H at reconstructed rutile TiO2(011)-2×1 surface with both H's at the top-oxygen (a and b), and one at top-oxygen and the other at bridging-oxygen (c−e).

of a single H atom at the three different surface O sites, i.e., top, bridging, and trough sites, was computed. In Figure 5 the optimized adsorption structures for hydrogen on the three different surface oxygen sites are illustrated. The calculated adsorption energies for these configurations are also given in Figure 5. The highest adsorption energy is for that on the topO site with adsorption energy of −0.03 eV. The other two surface sites give significantly lower adsorption energies of ∼ − 0.5 eV. Therefore these calculations favor the adsorption of hydrogen on the top O sites, in agreement with our STM studies. However, the calculated adsorption energies still indicate a very weak adsorption; in fact the negative value indicates that it is energetically unfavorable with respect to H2 occurrence in gas phase. This may be contrasted to the adsorption of hydrogen on TiO2(110) surface where we determined a small positive adsorption energy. This clearly shows that, as expected, the reconstructed TiO2(011)-2×1

hydroxyl covered surface and (b) the same sample after 90 min UV-illumination in UHV at room temperature. It is evident that after UV illumination all of the hydroxyls are removed from the terraces, indicating their desorption from the surface. Furthermore, no evidence of oxygen vacancy formation can be found, suggesting that hydrogen desorbs as H2 rather than reacting with lattice oxygen to form water. Furthermore, our UV-irradiation experiments on a hydroxyl covered (110) surface confirmed the previous reports18 that hydroxyls remain at the surface. Therefore these two surfaces exhibit a pronounced difference in the photolysis of hydrogen.

4. COMPUTATIONAL RESULTS To verify some of the experimental observations and to gain more detail of the thermodynamic and kinetic pathways we performed DFT calculations for hydrogen adsorption on the reconstructed TiO2(011)-2×1 surface. The adsorption energy 20442

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Figure 7. Calculated energy profile for H diffusion from top-oxygen A to trough-oxygen I via bridging-oxygen F. All of the calculated elementary diffusion steps are illustrated by arrows in the surface structure on the right. The barriers for each step along the most favorable pathway are given in the profile and the others are listed. The adsorption state of H at top-oxygen A is set to zero in the energy profile.

Figure 8. Calculated energy profile for H diffusion from trough-oxygen I into the bulk (oxygen V1). The elementary steps along the most favorable pathway are illustrated by arrows, and the corresponding barriers are given in the profile. The state of H adsorption at trough-oxygen I has the same energetic level as that in Figure 7.

performed systematic calculations to determine the complete pathway for the H to move from the top-oxygen into the bulk area. The overall pathway can be divided into three parts. Part I refers to the diffusion of H from the most favorable top-oxygen to the trough-oxygen, which is close to the opening of the [001] channel into the bulk (see Figure 1b). Part II refers to the diffusion of H from the trough oxygen at the opening of [001] channel into the subsurface region. Then, Part III refers to the diffusion of H within the bulk through the [001] channel, which was calculated using a rutile TiO2 bulk model. In Figure 7, we illustrate the H diffusion pathway in Part I, together with the calculated energy profile. As one can see, the pathway for H to move from top-oxygen A to B just corresponds to the general diffusion process of H at the (011)-2×1 surface. The calculated barrier of 0.75 eV also suggests that such diffusion may barely occur at room temperature. In order to move to the trough-oxygen I, the H at top-oxygen first needs to transfer to the nearby bridgingoxygen F. Three pathways, i.e., OA→OF, OB→OF and OC→ OF, were calculated, and they give barriers of 1.20, 1.14, and 1.16 eV, respectively. Apparently, OA → OF diffusion for H is

surface is much less reactive than the (110) surface toward hydrogen adsorption. We also investigated if the adsorption energy and/or the adsorption site preference is altered for higher hydrogen coverage. Figure 6 shows different coadsorption configurations for two H atoms and their corresponding adsorption energies per hydrogen atom. One can see that the adsorption energy is further reduced compared to that of a single hydrogen, and the top-oxygen sites are still preferred for both hydrogen atoms. In addition, these calculated energetics also clearly indicate that H's at the top-oxygen tend to avoid staying close to each other, which is again in line with STM results. The negative adsorption energy of hydrogen on the (011)2×1 surface may suggest that hydrogen would recombine and desorb from the surface as H2 if energy barriers were small enough. To assess this possibility we estimated the reaction barrier for a coupling reaction of two adjacent hydroxyls to H2. The calculated barrier is in excess of 1.9 eV, and therefore, this reaction could only occur at extremely high temperatures. In order to compare the possibility for hydrogen diffusion into the bulk with that of recombination at the surface, we 20443

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calculations (U = 4.2 eV) for the adsorption energies and diffusion barriers.40,41 These results show only slight difference compared to those obtained with GGA in the above and, most importantly, nearly no change with respect to the relative stabilities of various H adsorption states or preference for different diffusion pathways. For example, the GGA+U calculations give adsorption energies of −0.06 eV for a single H at the top O (Figure 5a) and −0.14 eV for two H at two separate top O (Figure 6b), which are −0.03 and −0.10 eV, respectively, from GGA calculations. At the same time, for the I → II-2 process (Figure 8), the barrier obtained from GGA calculations is 0.46 eV and it is 0.56 eV from GGA+U; in addition, the B → F process (Figure 7) is still the most difficult with the calculated barrier of 1.21 eV (1.14 eV without U). In addition, we also calculated the H2 adsorption in the bulk to test the possibility of H coupling during their diffusion. However, such H2 bulk-trapping state was estimated to be ∼1.7 eV less stable than that with separately adsorbed H in the bulk. This suggests that only atomic H is present in the rutile TiO2 bulk. Though H2O desorption, together with the occurrence of surface O vacancies, was not observed in the high-temperature STM images, we still calculated this process with DFT. By taking the coadsorption system with two H adsorbed at neighboring top oxygen (Figure 6a) as the initial state, we located the transition state for the formation of a H2O and estimated the barrier to be 0.78 eV. The final state formed directly following the transition state contains a H2O adsorbed at the O vacancy, and its adsorption energy was estimated to be close to 1 eV. Therefore, the energy profile for the whole process of H2O formation and desorption is very similar to that at rutile TiO2(110) reported in a recent work.18 Accordingly, we may expect that this process is determined by thermodynamics only, and by using the same analysis procedure as that employed by Du et al., we also estimated it to be slightly exothermic above 500 K.

slightly more difficult because the OA−OF distance is longer than those of OB−OF and OC−OF. From the bridging-oxygen F, the H only needs to overcome a small barrier of 0.61 eV to get to the trough-oxygen I, whereas the barrier for it to move to another neighboring trough-oxygen H is as high as ∼2.3 eV. The trough-oxygen site can be taken as the entrance to the [001] channel. For the Part II pathway, we calculated all the possible elementary steps for the H to move from the troughoxygen I through the [001] channel to the area around two layers below the surface (see Figure 8). The most favorable pathway containing continuous H transfer from one O to another is illustrated in Figure 8, together with the calculated energy profile. As one can see, the highest barrier actually comes from the diffusion of H from one second-layer O3c to the neighboring O3c at the same level, which is ∼0.9 eV. For the barriers of other elementary diffusion steps, they are all below 0.5 eV. Finally, for the H diffusion in the [001] channel of the bulk, it has been widely studied by various methods and believed to be an easy process.38,39 In this work, we also calculated different possible pathways for the H to move between neighboring lattice oxygen in the channel. In Figure 9, we illustrate the

5. DISCUSSION The interpretation of empty state STM images of the rutile TiO2(011)-2×1 surface has been aided by the observation of the location of hydrogen adsorbates. The location of the hydroxyl can be used as a marker of a single surface top-oxygen site, which then allows assigning of the positions of the location of the protrusions observed on the clean surface. From this we conclude that under common imaging conditions the Ti-sites are predominantly imaged, which is similar to the well established imaging of the (110) surface42 and in agreement with recently refined simulated STM images.30 The main emphasis of this work is, however, on understanding of hydrogen adsorption and reactions on TiO2 surfaces, which is of fundamental importance to better describing processes such as photolysis of water. The (011)2×1 surface can act as a model surface that allows investigating how hydrogen reacts on a surface that bonds hydrogen only weakly and comparing it to more strongly interacting TiO2 surfaces such as the frequently studied rutile (110) surface. The difference in the adsorption energy of hydrogen on these two surfaces has its origin in the surface structure. While on the (110) surface hydrogen can terminate the “dangling” bond of the 2-fold top oxygen, the reconstruction of the (011)-2×1 surface already minimizes unsaturated bonds and thus is much less reactive.43 Contrary to a naive expectation that the weaker interacting surface should favor recombinative desorption of

Figure 9. Structure of bulk rutile TiO2 from the view against the [001] direction with the characteristic Ti4O4 square unit shown in ball-andstick mode. The elementary diffusion steps for H between neighboring O in the unit are illustrated by arrows. O1 and 3 have the same height, which is higher than that of O2 and 4.

structure of the bulk TiO2 with a characteristic Ti4O4 square unit specified in ball-and-stick mode. As one can see, the two O at the neighboring sides of the square have different relative positions along the [001] direction, while those at two opposite sides have the same height along [001]. According to our calculations, the H diffusion between O at these two sides can give relatively small barriers, which are 0.50 eV between two neighboring sides and only 0.29 eV between two opposite sides. These results suggest that the transfer of H between two O at neighboring sides, which corresponds to its movement along the [001] directions, and the transfer of it between two O at opposite sides, which only contributes to its oscillation at the same level, would be the dominant for H in rutile bulk even under mild conditions. It needs to be mentioned that H adsorption at TiO2 also induces its reduction, and considering the deficiency of regular GGA functionals to accurately describe the (partially occupied) localized Ti 3d states we also performed testing GGA+U 20444

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surface and desorption of H2. An adsorbed hydrogen atom donates its electron to lattice titanium and occupies Ti-3d states.31 In our experiments we cannot distinguish if the UVexcitation and subsequent hydrogen desorption is due to a direct excitation of the Ti-3d state and back-donation to the adsorbed hydrogen or if it is due to exciton formation in the TiO2 bulk and subsequent charge transfer at the surface to adsorbed hydrogen of these excited charge carriers. However, it is interesting to point out that experiments on the (110) surface suggested that similar UV irradiation as in our experiments does not lead to hydrogen desorption18 and therefore this is another interesting distinction between these two rutile surfaces. Since our DFT calculations indicate that hydrogen is only very weakly adsorbed on the (011)-2×1 surface, electron excitation may simply modify the reaction barrier for hydrogen recombination to occur. On the (110) surface, on the other hand the much stronger adsorption of hydrogen appears to prevent recombination of two adsorbed hydrogen atoms even if one hydroxyl is in an excited state. While these observations suggest that the (011) surface may have better photocatalytic properties than the (110) surface for hydrogen production, this should be considered with caution, because the 2×1 reconstruction may not be intact under nonvacuum conditions.14,43,44 Nevertheless, our study demonstrates that hydrogen production by photolysis sensitively depends on the TiO2 surface structure and implies that hydrogen photolysis may be enhanced by reducing the binding energy between hydrogen and the metal oxide photocatalyst.

hydrogen, our experiments indicate that thermal excitation causes diffusion of hydrogen into the bulk. Our DFT simulations of the kinetic barriers support the diffusion into the bulk as the path with lower kinetic barriers. The ratelimiting barrier for hydrogen diffusion into the bulk was estimated to be only 1.14 eV, which is significantly less than the barrier for hydrogen recombination reaction of 1.9 eV. Although the adsorption energy of hydrogen at the surface of (011)-2×1 is small, it is even smaller in the bulk. Thus the adsorption energy by itself does not provide a sufficient driving force for hydrogen diffusion into the bulk. However, the more relevant thermodynamic property is the Gibbs free energy and the entropic term may play a decisive role in determining if hydrogen stays at the surface or diffuses into the bulk. The configuration entropy, i.e., the number of adsorption sites, is obviously much larger in the bulk than at the surface. In addition, H in the bulk gives much weaker H-lattice interaction as well as strong perturbation to the lattice structures, compared to hydrogen adsorbed at the surface. Both of these effects increase the vibrational entropy for the corresponding hydrogen and lattice vibrations. The increased entropy of hydrogen in the bulk may tip the balance of the Gibbs free energy in favor for hydrogen diffusion into the bulk at elevated temperatures and thus explain the experimental observation of a reversible hydrogen diffusion from the surface to the bulk in a temperature cycle from room temperature to ∼500 K and back. The experimental observation that hydrogen diffuses into the bulk already below 500 K also allows us to explain the lack of abstraction of lattice oxyen and formation of H2O. Although our computations found that similar to the rutile TiO2(110) surface H2O formation and desorption is barrierless and becomes exothermic above 500 K, it appears not to take place because it competes with hydrogen diffusion into the bulk. In other words, at the temperature water formation would become exothermic most of the hydrogen has diffused into the bulk. Thus we believe that the difference for water formation between the (110) and (011)-2×1 surface has its origin in the larger adsorption energy of hydrogen on the (110) surface that allows hydrogen to remain at the surface to higher annealing temperatures at which water formation may occur. In addition to these thermodynamic arguments, kinetic diffusion pathways related to the crystal structure of rutile also favors hydrogen diffusion into the bulk for the (011) surface compared to the (110) surface. As can be seen in Figure 1b, the straight [001] diffusion-channel is directly below the top oxygen sites, and most importantly, each top oxygen ‘hosts’ just one channel. This implies that every H coming from the top oxygen can diffuse through a single [001] channel deep into the bulk without any interference from other hydrogen atoms within the same channel. This is in contrast to the (110) surface where the [001] channels are parallel to the surface. Thus in an initial diffusion step of hydrogen from the surface into the bulk all the hydrogen atoms on bridging oxygen row have only a single [001] channel available. This implies strong interferences between the hydrogen atoms and a small amount of subsurface hydrogen can block other surface hydrogen from transferring into the subsurface channel. This blocking of hydrogen diffusion into the bulk may promote their reaction with top O for water formation, especially when the H coverage is high at the surface.18 While barriers for thermal desorption of H2 are higher than those for diffusion into the bulk, electron excitation by UV illumination seems to enable hydrogen recombination at the

6. CONCLUSIONS Hydrogen diffusion and reactions on TiO2 surfaces plays an important role in thermal and photo assisted chemical transformation reactions. Our results indicate that bulk diffusion plays an important role in understanding these surface processes. The balance between surface and bulk adsorption of hydrogen has been shown to be different for the rutile (110) and (011)-2×1 surfaces. Weak hydrogen adsorption and open bulk-diffusion channels on the (011)-2×1 favor thermally activated bulk diffusion at mild temperatures (below 500 K) and thus preventing water formation by lattice oxygen abstraction from the surface. Thermally activated hydrogen recombination of adsorbed hydrogen is not observed due to a large kinetic barrier, however, we find that UV-excitation can induce H2 desorption on the (011)-2×1 surface. This is contrasted to reports on the (110) surface where such desorption is not observed, suggesting that weak hydrogen adsorption facilitates hydrogen photolysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Author Contributions §

Authors contributed equally to this manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The USF group acknowledges financial support from the Department of Energy (DE-FG02-09ER16082) and the National Science Foundation (CHE-0840547 and CBET1033000). The ECUST group is thankful for financial support from National Basic Research Program (2010CB732300 and 20445

dx.doi.org/10.1021/jp3064678 | J. Phys. Chem. C 2012, 116, 20438−20446

The Journal of Physical Chemistry C

Article

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2011CB808505) and National Natural Science Foundation of China (21073060). X.Q.G. also thanks Shanghai Institutions of Higher Learning for the program for professor of special appointment (Eastern Scholar). We thank Dr. Teobaldi for sharing his results (ref 30) prior to publication.

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dx.doi.org/10.1021/jp3064678 | J. Phys. Chem. C 2012, 116, 20438−20446